1. Field of the Invention
This application relates to Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 DSSS Offset-QPSK devices, and more specifically allowing an existing IEEE 802.15.4 DSSS Offset-QPSK system to transfer Offset-QPSK modulation signal into minimum-shift keying (MSK) modulation signal, then deliver the MSK modulation signal without DSSS while increasing the rate of payload data transmission.
2. Description of the Prior Art
The IEEE 802.15.4 specification defines the physical and media access protocols for 2.4 GHz Industrial, Scientific, and Medical (ISM) radio bands for wireless network applications. It provides a direct-sequence spread spectrum (DSSS) Offset-Quadrature Phase-shift keying (Offset-QPSK) modulation network to support 250 Kbps data transmission.
There are two main steps in IEEE 802.15.4 baseband transmission; the first step is DSSS and the second step is Offset-QPSK modulation with half-sine pulse shaping. The DSSS is a 4:32 spreading rate scheme and outputs 32 digits of pseudo spreading code for each 4 digits of input bits. When co-operating with a 250 Kbps input data rate, the DSSS spread input signal to a 2 Mega chips rate signal. The Offset-QPSK modulation used in this system is a variant of QPSK modulation in that the Q-Phase signal half chip period is offset (delayed) from the I-Phase signal, then half-sine pulse shaping is performed on the I-Phase and Q-Phase signals. This offsetting and shaping action can avoid a zero-crossing in constellation mapping of quadrature modulation.
Referring to FIG. 1 upper half region, the frame structure of an IEEE 802.15.4 Offset-QPSK transmitted package consists of a preamble, a delimiter (SFD), frame length bits (PHR), and payload data. The PHR has 8 bits, 7 bits for indicating frame length and the 8th bit is not used. Therefore, the maximum payload data length is 127 bytes.
In a typical standard IEEE 802.15.4 network, every station has to transmit at the above mentioned data rate and protocol in order to be certified and communicate with each other. However, there are applications where multi-rate transmission and on fly detection are desired.
Some applications may desire high date rate transmission bandwidth but are limited by system specification even the received signal quality or network link quality is satisfied for transmission without spreading gain. Applying the proposed multi-rate transmission and on-fly detection can provide adoptive transmission rate for more applications.